Method of carrying out maintenace of multi-disc brake arrangement

ABSTRACT

A multi-disc brake suitable for an aircraft has a stack of rotor and stator discs in which the discs of a set at one end of the stack each have a greater allowance for wear than the discs of a set at the other end of the stack whereby the group of discs are removed together from one end of the stack as a fully worn group at a brake servicing operation. An unworn group of discs is added to the other end of the stack.

This invention relates to multi-disc brakes, such as are used inaircraft brake systems.

In recent years the rotor and stator discs for aircraft brakesincreasingly have been made from carbon-carbon composite material, thismaterial providing both the frictional surfaces of the discs and theirstructural integrity. The rotor discs are formed with slots at theirouter peripheries to receive drive keys attached to the aircraft wheel,and the stator discs are correspondingly slotted at their innerperipheries to engage splines on a stationary torque tube.

In order to provide allowance for wear of the braking surfaces, theinitial thicknesses of the discs have to be greater than may be requiredby their structural function in transmitting torque from theirrespective keys or splines. Thus the axial length, and weight, of thedisc stack is initially greater than the optimum.

Further, in normal practice discs of a given type (i.e. rotors, singlestators or double stators) are all worn concurrently from their originalthickness to their fully worn thickness, which means that the initialsize of a brake may be unnecessarily large.

One object of the present invention is to provide a multi-disc carbonbrake which may be more economically operated, and a second object is toenable such a brake to be made lighter and more compact thanconventional brakes of similar performance and the same expected discwear life.

According to the invention, a multi-disc brake comprises a stack ofinterleaved rotor and stator discs arranged about a common axis, athrust device provided at one end of the stack and a reaction member atthe other end to create axial brake-applying pressure on the stack inoperation of the thrust device and thereby generate friction betweenrubbing faces of adjacent rotor and stator discs wherein the discs arearranged so that the wear allowance of a set of discs at one end of thestack is greater than the wear allowance of a set of discs at the otherend of the stack, whereby during a brake service operation followingwear a fully worn group of discs may be removed from the said other endand the remaining discs may be moved axially to a new position towardssaid other end of the stack, an unworn group of discs being added at thesaid one end of the stack.

Preferably, the arrangement is such that upon replacement of worn discsthe remaining discs in the stack are maintained in mutual relationshipso that partly worn mating faces of adjacent discs remain in contactthroughout their entire wear lives.

The invention is based upon the concept that by increasing the wearallowance of the discs progressively from one end of the stack to theother it is possible to operate a schedule of maintenance in whichcertain discs may be moved progressively through the stack, sustainingwear at different positions, and becoming fully worn in a position at oradjacent the other end. In practice, the two end discs of an aircraftbrake are single stators, i e they do not rotate about the axis with thewheel and are worn on one side face only, the other side face eitherbeing in contact with a hydraulic thrust or reaction member for thisreason, it is not normally practicable to replace a `group` consistingof a single stator disc, and the minimum group size is thereforenormally two discs, a rotor disc and a stator disc.

One embodiment of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic axial cross-section showing part of an aircraftbrake assembly in its initial unworn state comprising a piston assemblyP, a reaction plate R, and a stack of interleaved stator and rotor discs1-9;

FIG. 2 is a similar view to FIG. 1 showing the brake in a worn conditionprior to a service operation, and

FIG. 3 is a similar view to FIG. 1 showing the brake re-assembledfollowing the service operation with worn discs removed and new discsinserted.

In the example illustrated in the drawings, carbon-carbon compositediscs 1-9 as shown in FIG. 1 are made with wear allowances as follows(where t is the normal wear allowance per wear face during a serviceperiod after which the disc position will be changed within the stack orthe disc will be removed from the brake and where s, r, and d are thefinal thicknesses of the single stators, rotors, and double statorsrespectively which are greater than or equal to the thicknessesnecessary to carry the torque loadings applied to the discs when thediscs are in the fully worn state.

    ______________________________________                                                Original Total Wear                                                           Disc     Allowance    Thickness after                                 Disc    Function Required     First Wear Period                               ______________________________________                                        1       single   1 × t  s                                                       stator                                                                2       rotor    2 × t  r                                               3       double   2 × t  d                                                       stator                                                                4       rotor    2 × t  r                                               5       double   3 × t  s + t                                                   stator                                                                6       rotor    4 × t  r + 2t                                          7       double   4 × t  d + 2t                                                  stator                                                                8       rotor    4 × t  r + 2t                                          9       single   4 × t  s + 3t                                                  stator                                                                ______________________________________                                    

The above example assumes that the wear allowances are equal for rotorsand stators.

A brake which is initially assembled with disc wear allowances asindicated in the above table is intended to be run in service until allof the wear allowances of the group comprising discs 1-4 have been used(this will normally occur simultaneously for all of discs 1-4 asdesigned in accordance with the table).

Discs 1-4 will then be removed as fully worn, and discs 5-9 moved alongthe axle to occupy the positions originally occupied by discs 1-5. Newdiscs 10-13 are added to occupy the positions previously held by discs6-9. The new discs 10-13 will have wear allowances similar to theoriginal discs 6-9, thus re-establishing the initial condition of thebrake. It will be noted that the special wear allowances for discs 1-5apply only to the original brake fitted to the aircraft, subsequent discreplacements always, in this example, entail the insertion of a standardpack of four discs similar to the original discs 6-9. It should be notedthat in order to maintain the torque-carrying capacity of discs 5, s +tshould not be less than d.

In the example described above the `group` of discs for replacementcomprises four discs : in other examples the number of discs for removalas a group may be different.

In an alternative example involving a 7-disc brake sets of three discsat each end will be provided with different wear allowances (consideringthe sets as a whole), and a group of four discs may be replaced at eachservice.

The construction and maintenance schedule of the brake described aboveprovides a light and compact brake having a good wear life betweenservice intervals.

A further important advantage of the brake in accordance with theinvention is that where adjacent discs have been run initially withtheir wear surfaces in mutual engagement these discs are retained incontact throughout the full service lives of these discs. The importanceof this feature stems from the wear patterns which the mutually engagingsurfaces develop during service such patterns on each disc conform tothose of the mating disc, and it is highly advantageous to retain themas a `friction pair`, giving better frictional characteristics thanwould be obtained by mating a worn surface with that of a new disc. (Inthe example described, the surfaces of discs 9 and 10 which make contactas shown in FIG. 3 are both initially unworn surfaces).

What we claim is:
 1. A method of carrying out the maintenance of amulti-disc brake of the kind comprising a stack of interleaved rotor andstator discs provided between a thrust device and a reaction member inan arrangement in which friction is generated between rubbing faces ofadjacent rotor and stator discs, said method comprising:selecting andarranging the rotor and stator discs so that the wear allowance of a setof discs at one end of the stack is less than the wear allowance of aset of discs at the other end of the stack; removing a fully worn groupof discs from said one end of the stack during a brake service operationfollowing wear; axially moving or shifting the remaining partially worndiscs of the stack to a new position towards said one end of the stack,and adding an unworn group of discs to the other end of the stack.
 2. Amethod according to claim 1 wherein when removing worn discs theremaining discs in the stack are maintained in a mutual relationship sothat mating faces of adjacent partly-worn discs remain in their originalmating relationship throughout their entire wear lives.
 3. A methodaccording to claim 1 wherein the thicknesses of the respective discs areselected such as to provide appropriate wear allowances in accordancewith the position to be occupied by the discs during their wear lives.4. A method according to claim 3 wherein the stack comprises nine discsand the wear allowances of the respective discs are such as to enablethe brake discs to be replaced in groups of four discs.
 5. A methodaccording to claim 4 wherein the stack comprises seven discs and thewear allowances of the respective discs are such as to enable the brakediscs to be replaced in groups of four discs.
 6. A method according toclaim 1 and comprising providing discs of a kind in which the frictionsurfaces of the discs and their structural integrity are provided bycarbon-carbon composite material.